Aluminum Secondary Battery Having a High-Capacity and High-Rate Capable Cathode and Manufacturing Method

ABSTRACT

Provided is an aluminum secondary battery comprising an anode, a cathode, a porous separator electronically separating the anode and the cathode, and an electrolyte in ionic contact with the anode and the cathode to support reversible deposition and dissolution of aluminum at the anode, wherein the anode contains aluminum metal or an aluminum metal alloy as an anode active material and the cathode comprises a layer of recompressed exfoliated graphite or carbon material that is oriented in such a manner that the layer has a graphite edge plane in direct contact with the electrolyte and facing the separator. Typically, this graphite edge plane is substantially parallel to the separator layer plane. Such an aluminum battery delivers a high energy density, high power density, and long cycle life.

FIELD OF THE INVENTION

The present invention relates generally to the field of rechargeablealuminum battery and, more particularly, to a high-capacity andhigh-rate capable cathode layer containing a new group of oriented,recompressed exfoliated graphite or carbon materials and a method ofmanufacturing this cathode layer and the aluminum battery.

BACKGROUND OF THE INVENTION

Historically, today's most favorite rechargeable energy storagedevices—lithium-ion batteries—was actually evolved from rechargeable“lithium metal batteries” that use lithium (Li) metal as the anode and aLi intercalation compound (e.g. MoS₂) as the cathode. Li metal is anideal anode material due to its light weight (the lightest metal), highelectronegativity (−3.04 V vs. the standard hydrogen electrode), andhigh theoretical capacity (3,860 mAh/g). Based on these outstandingproperties, lithium metal batteries were proposed 40 years ago as anideal system for high energy-density applications.

Due to some safety concerns of pure lithium metal, graphite was laterimplemented as an anode active material in place of the lithium metal toproduce the current lithium-ion batteries. The past two decades havewitnessed a continuous improvement in Li-ion batteries in terms ofenergy density, rate capability, and safety. However, the use ofgraphite-based anodes in Li-ion batteries has several significantdrawbacks: low specific capacity (theoretical capacity of 372 mAh/g asopposed to 3,860 mAh/g for Li metal), long Li intercalation time (e.g.low solid-state diffusion coefficients of Li in and out of graphite andinorganic oxide particles) requiring long recharge times (e.g. 7 hoursfor electric vehicle batteries), inability to deliver high pulse power,and necessity to use pre-lithiated cathodes (e.g. lithium cobalt oxide,as opposed to cobalt oxide), thereby limiting the choice of availablecathode materials. Further, these commonly used cathode active materialshave a relatively low lithium diffusion coefficient (typicallyD˜10⁻¹⁶-10⁻¹¹ cm²/sec). These factors have contributed to one majorshortcoming of today's Li-ion batteries—a moderate energy density(typically 150-220 Wh/kg_(cell)), but extremely low power density(typically <0.5 kW/kg).

Supercapacitors are being considered for electric vehicle (EV),renewable energy storage, and modern grid applications. Thesupercapacitors typically operate on using porous electrodes havinglarge surface areas for the formation of diffuse double layer charges.This electric double layer capacitance (EDLC) is created naturally atthe solid-electrolyte interface when voltage is imposed. This impliesthat the specific capacitance of an EDLC-type supercapacitor is directlyproportional to the specific surface area of the electrode material,e.g. activated carbon. This surface area must be accessible by theelectrolyte and the resulting interfacial zones must be sufficientlylarge to accommodate the EDLC charges.

This EDLC mechanism is based on ion adsorption on surfaces of anelectrode. The required ions are pre-existing in a liquid electrolyteand do not come from the opposite electrode. In other words, therequired ions to be deposited on the surface of a negative electrode(anode) active material (e.g., activated carbon particles) do not comefrom the positive electrode (cathode) side, and the required ions to bedeposited on the surface of a cathode active material do not come fromthe anode side. When a supercapacitor is re-charged, local positive ionsare deposited close to a surface of a negative electrode with theirmatting negative ions staying close side by side (typically via localmolecular or ionic polarization of charges). At the other electrode,negative ions are deposited close to a surface of this positiveelectrode with the matting positive ions staying close side by side.Again, there is no exchange of ions between an anode active material anda cathode active material.

In some supercapacitors, the stored energy is further augmented bypseudo-capacitance effects due to some local electrochemical reactions(e.g., redox). In such a pseudo-capacitor, the ions involved in a redoxpair also pre-exist in the same electrode. Again, there is no exchangeof ions between the anode and the cathode.

Since the formation of EDLC does not involve a chemical reaction or anexchange of ions between the two opposite electrodes, the charge ordischarge process of an EDLC supercapacitor can be very fast, typicallyin seconds, resulting in a very high power density (typically 2-8kW/Kg). Compared with batteries, supercapacitors offer a higher powerdensity, require no maintenance, offer a much higher cycle-life, requirea very simple charging circuit, and are generally much safer. Physical,rather than chemical, energy storage is the key reason for their safeoperation and extraordinarily high cycle-life.

Despite the positive attributes of supercapacitors, there are severaltechnological barriers to widespread implementation of supercapacitorsfor various industrial applications. For instance, supercapacitorspossess very low energy densities when compared to batteries (e.g., 5-8Wh/kg for commercial supercapacitors vs. 20-40 Wh/Kg for the lead acidbattery and 50-100 Wh/kg for the NiMH battery). Modern lithium-ionbatteries possess a much higher energy density, typically in the rangeof 150-220 Wh/kg, based on the cell weight.

Secondary batteries based on various charge-discharge principles otherthan lithium ions have been proposed. Among them, some attention hasbeen paid to aluminum secondary batteries based on thedeposition-dissolution reaction of aluminum (Al) at the anode. Aluminumhas a high ionization tendency and is capable of three-electron redoxreactions, which can potentially enable an aluminum battery to deliver ahigh capacity and high energy density.

The abundance, low cost, and low flammability of Al, and its ability toundergo three-electron redox imply that rechargeable Al-based batteriescould in principle offer cost-effectiveness, high capacity and safety.However, the rechargeable Al batteries developed over the past 30 yearshave failed to make it to the marketplace. This has been likely due toproblems such as cathode material disintegration, low cell dischargevoltage (e.g. 0.55V), a capacitive behavior without a discharge voltageplateau (e.g. 1.1-0.2 V), and short cycle life (typically <100 cycles)with rapid capacity decay (by 26-85% over 100 cycles), low cathodespecific capacity, and low cell-level energy density (<50 Wh/kg).

For instance, Jayaprakash reports an aluminum secondary battery thatshows a discharge curve having a plateau at 0.55 V [Jayaprakash, N.,Das, S. K. & Archer, L. A. “The rechargeable aluminum-ion battery,”Chem. Commun. 47, 12610-12612 (2011)]. A rechargeable battery having anoutput voltage lower than 1.0 volt has a limited scope of application.As a point of reference, alkaline battery has an output voltage of 1.5volts and a lithium-ion battery has a typical cell voltage of 3.2-3.8volts. Furthermore, even with an initial cathode specific capacity ashigh as 305 mAh/g, the energy storage capability of the cathode isapproximately 0.55 V×305 mAh/g=167.75 Wh/kg based on the cathode activematerial weight alone (not based on the total cell weight). Thus, thecell-level specific energy (or gravimetric energy density) of thisAl—V₂O₅ cell is approximately 167.75/3.6=46.6 Wh/kg (based on the totalcell weight).

(As a point of reference, a lithium-ion battery having a lithium ironphosphate (LFP) as the cathode active material (having a theoreticalspecific capacity of 170 mAh/g) delivers an output voltage of 3.2 voltsand an energy storage capability of 3.2 V×170 mAh/g=544 Wh/kg (based onthe LFP weight only). This cell is known to deliver a cell-level energydensity of approximately 150 Wh/kg. There is a reduction factor of544/150=3.6 to convert a cathode active material weight-based energydensity value to a total cell weight-based energy density value in thisbattery system.)

As another example, Rani reports an aluminum secondary battery using alightly fluorinated natural graphite as the cathode active materialhaving an output voltage varying from 0.2 volts to 1.1 volts [Rani, J.V., Kanakaiah, V., Dadmal, T., Rao, M. S. & Bhavanarushi, S.“Fluorinated natural graphite cathode for rechargeable ionic liquidbased aluminum-ion battery,” J. Electrochem. Soc. 160, A1781-A1784(2013)]. With an average voltage of approximately 0.65 volts and adischarge capacity of 225 mAh/g, the cell delivers an energy storagecapability of 0.65×225=146.25 Wh/kg (of the cathode active materialweight only) or cell-level specific energy of 146.25/3.6=40.6 Wh/kg(based on the total cell weight).

As yet another example, Lin, et al. reports an aluminum-graphite foamcell that exhibits a plateau voltage near 2 volts and an output voltageof 70 mAh/g [Lin M C, Gong M, Lu B, Wu Y, Wang D Y, Guan M, Angell M,Chen C, Yang J, Hwang B J, Dai H., “An ultrafast rechargeablealuminum-ion battery,” Nature. 2015 Apr. 16; 520 (7547):325-8]. Thecell-level specific energy is expected to be approximately70×2.0/3.6=38.9 Wh/kg. As a matter of fact, Lin, et al. has confirmedthat the specific energy of their cell is approximately 40 Wh/kg.

Clearly, an urgent need exists for new cathode materials for an aluminumsecondary battery that provide proper discharge voltage profiles (havinga high average voltage and/or a high plateau voltage during discharge),high specific capacity at both high and low charge/discharge rates (notjust at a low rate), and long cycle-life. Hopefully, the resultingaluminum battery can deliver some positive attributes of asupercapacitor (e.g. long cycle life and high power density) and somepositive features of a lithium-ion battery (e.g. moderate energydensity). These are the main objectives of the instant invention.

SUMMARY OF THE INVENTION

The invention provides a cathode or positive electrode layer for analuminum secondary battery (rechargeable aluminum battery oraluminum-ion battery) and an aluminum secondary battery containing sucha cathode layer.

In some preferred embodiments, the invented aluminum secondary batterycomprises an anode, a cathode, a porous separator electronicallyseparating the anode and the cathode, and an electrolyte in ioniccontact with the anode and the cathode to support reversible depositionand dissolution of aluminum at the anode, wherein the anode containsaluminum metal or an aluminum metal alloy as an anode active materialand the cathode comprises a layer of recompressed exfoliated graphite orcarbon material that is oriented in such a manner that the layer has agraphite edge plane in direct contact with the electrolyte (to readilyadmit ions from the electrolyte and release ions into electrolyte) andfacing the separator (so that the ions permeating through the porousseparator can readily enter the inter-flake spaces near the edge plane).

In certain embodiments, the layer of recompressed exfoliated graphite orcarbon material has a physical density from 0.5 to 1.8 g/cm³ and hasmeso-scaled pores having a pore size from 2 nm to 50 nm. In somepreferred embodiments, the layer of recompressed exfoliated graphite orcarbon material has a physical density from 1.1 to 1.8 g/cm³ and haspores having a pore size from 2 nm to 5 nm. In certain embodiments, theexfoliated graphite or carbon material has a specific surface area from20 to 1,500 m²/g. Preferably, the specific surface area is from 20 to1,000 m²/g, more preferably from 20 to 300 m²/g.

Preferably, the electrolyte also supports reversible intercalation andde-intercalation of ions (cations, anions, or both) at the cathode. Thealuminum alloy preferably contains at least 80% by weight Al element inthe alloy (more preferably at least 90% by weight). There is norestriction on the type of alloying elements that can be chosen.Preferably, the alloying elements for Al are Si, B, Mg, Ti, Sc, etc.

This aluminum secondary battery can further comprise an anode currentcollector supporting the aluminum metal or aluminum metal alloy orfurther comprise a cathode current collector supporting the cathodeactive layer. The current collector can be a mat, paper, fabric, foil,or foam that is composed of conducting nano-filaments, such as graphenesheets, carbon nanotubes, carbon nano-fibers, carbon fibers, graphitenano-fibers, graphite fibers, carbonized polymer fibers, or acombination thereof, which form a 3D network of electron-conductingpathways. The high surface areas of such an anode current collector notonly facilitate fast and uniform dissolution and deposition of aluminumions, but also act to reduce the exchange current density and, thus,reduce the tendency to form metal dendrites that otherwise could causeinternal shorting.

The exfoliated carbon or graphite material is preferably selected from athermally exfoliated product of meso-phase pitch, meso-phase carbon,meso carbon micro-beads (MCMB), coke particles, expanded graphiteflakes, artificial graphite particles, natural graphite particles,highly oriented pyrolytic graphite, soft carbon particles, hard carbonparticles, multi-walled carbon nanotubes, carbon nano-fibers, carbonfibers, graphite nano-fibers, graphite fibers, carbonized polymerfibers, or a combination thereof.

The above-listed carbon/graphite material may be subjected to aninter-planar spacing expansion treatment, followed by a thermalexfoliation and recompression. The expansion treatment is conducted toincrease the inter-planar spacing between two graphene planes in agraphite crystal, from a typical value of 0.335-0.36 nm to a typicalvalue of 0.43-1.2 nm for the main purpose of weakening the van der Waalsforces that hold neighboring graphene planes together. This would makeit easier for subsequent thermal exfoliation. This expansion treatmentincludes an oxidation, fluorination, bromination, chlorination,nitrogenation, intercalation, combined oxidation-intercalation, combinedfluorination-intercalation, combined bromination-intercalation, combinedchlorination-intercalation, or combined nitrogenation-intercalation ofthe graphite or carbon material. The above procedure is followed by athermal exfoliation without constraint. Unconstrained thermalexfoliation typically results in exfoliated graphite/carbon worms thathave inter-flake pores having an average size from 20 nm to 50 μm (moretypically from 100 nm to 10 μm). The exfoliated graphite/carbon wormsare then compressed to produce a layer or block of recompressedexfoliated carbon or graphite material that is oriented in such a mannerthat the layer has a graphite edge plane in direct contact with theelectrolyte and facing or contacting the separator. The recompressedexfoliated carbon or graphite material typically has a physical densityfrom 0.5 to 1.8 g/cm³ and has meso-scaled pores having a pore size from2 nm to 50 nm.

Due to the expansion treatments, the carbon or graphite material cancontain a non-carbon element selected from oxygen, fluorine, chlorine,bromine, iodine, nitrogen, hydrogen, or boron.

In the invented aluminum secondary battery, the electrolyte may beselected from an aqueous electrolyte, organic electrolyte, molten saltelectrolyte, ionic liquid electrolyte, or a combination thereof. Apolymer may be added to the electrolyte. Preferably, the electrolytecontains an aluminum salt such as, AlF₃, AlCl₃, AlBr₃, AlI₃,AlF_(x)Cl_((3-x)), AlBr_(x)Cl_((3-x)), AlI_(x)Cl_((3-x)), or acombination thereof, wherein x is from 0.01 to 2.0. Mixed aluminumhalides, such as AlF_(x)Cl_((3-x)), AlBr_(x)Cl_((3-x)),AlI_(x)Cl_((3-x)), can be readily produced by brominating, fluorinating,or iodizing AlCl₃ to a desired extent; for instance at 100-350° C. for1-24 hours.

Preferably, the electrolyte contains an ionic liquid that contains analuminum salt mixed with an organic chloride selected fromn-butyl-pyridinium-chloride (BuPyCl),1-methyl-3-ethylimidazolium-chloride (MEICl),2-dimethyl-3-propylimidazolium-chloride, 1,4-dimethyl-1,2,4-triazoliumchloride (DMTC), or a mixture thereof.

In certain embodiments, the layer of carbon or graphite materialoperates as a cathode current collector to collect electrons during adischarge of the aluminum secondary battery and wherein the batterycontains no separate or additional cathode current collector.

The cathode active layer of graphite may further comprise anelectrically conductive binder material which bonds particles or fibersof the carbon or graphite material together to form a cathode electrodelayer. The electrically conductive binder material may be selected fromcoal tar pitch, petroleum pitch, meso-phase pitch, a conducting polymer,a polymeric carbon, or a derivative thereof.

Typically, the invented aluminum secondary battery has an averagedischarge voltage no less than 1 volt (typically and preferably >1.5volts) and a cathode specific capacity greater than 200 mAh/g(preferably and more typically >300 mAh/g, and most preferably >400mAh/g), based on a total cathode active layer weight.

Preferably, the aluminum secondary battery has an average dischargevoltage no less than 2.0 volts and a cathode specific capacity greaterthan 100 mAh/g based on a total cathode active layer weight (preferablyand more typically >300 mAh/g, and most preferably >400 mAh/g).

The present invention also provides a cathode active layer for analuminum secondary battery. The cathode active layer comprises anexfoliated graphite or carbon material having inter-flake pores from 2nm to 10 μm in pore size. Preferably, the exfoliated carbon or graphitematerial is a thermal exfoliation product of meso-phase pitch,meso-phase carbon, meso carbon micro-beads (MCMB), coke particles,expanded graphite flakes, artificial graphite particles, naturalgraphite particles, highly oriented pyrolytic graphite, soft carbonparticles, hard carbon particles, multi-walled carbon nanotubes, carbonnano-fibers, carbon fibers, graphite nano-fibers, graphite fibers,carbonized polymer fibers, or a combination thereof, wherein the carbonor graphite material has an original inter-planar spacing d₀₀₂ from 0.27nm to 0.42 nm prior to an expansion treatment and the inter-planarspacing d₀₀₂. is increased to a range from 0.43 nm to 1.2 nm after theexpansion treatment and wherein the carbon or graphite material issubsequently thermally exfoliated to have inter-flake pores from 2 nm to10 μm in pore size. The thermally exfoliated carbon or graphite is thenrecompressed.

The present invention also provides a method of manufacturing analuminum secondary battery. The method comprises: (a) providing an anodecontaining aluminum or an aluminum alloy; (b) providing a cathodecomprising a layer of recompressed exfoliated carbon or graphitematerial (recompressed graphite/carbon worms or recompressed expandedgraphite flakes, not graphene sheets); and (c) providing an electrolytecapable of supporting reversible deposition and dissolution of aluminumat the anode and reversible adsorption/desorption and/orintercalation/de-intercalation of ions at the cathode, wherein the layerof recompressed exfoliated carbon or graphite material is oriented insuch a manner that the layer has a graphite edge plane in direct contactwith the electrolyte and facing or contacting the separator. Typically,this graphite edge plane is substantially parallel to the porousseparator and, thus, can readily accommodate ions that migrate throughthe separator. Preferably, the electrolyte contains an aqueouselectrolyte, an organic electrolyte, a molten salt electrolyte, or anionic liquid.

The method can further include providing a porous network ofelectrically conductive nano-filaments to support the aluminum oraluminum alloy at the anode.

In the method, the step of providing a cathode preferably containssubjecting a carbon or graphite material to an expansion treatmentselected from an oxidation, fluorination, bromination, chlorination,nitrogenation, intercalation, combined oxidation-intercalation, combinedfluorination-intercalation, combined bromination-intercalation, combinedchlorination-intercalation, or combined nitrogenation-intercalation,followed by thermal exfoliation at a temperature from 100° C. to 2,500°C.

In certain preferred embodiments, the procedure of providing the cathodeincludes compressing exfoliated graphite or carbon using a wetcompression or dry compression to align constituent graphite flakes ofthe exfoliated graphite or carbon. The procedure can produce a layer orblock of graphitic structure having a flake edge plane being parallel tothe separator, enabling direct entry of ions from separator pores intointer-flake spaces with minimal resistance. Preferably, the procedure ofproviding the cathode includes compressing exfoliated graphite or carbonusing a wet compression to align constituent graphite flakes of theexfoliated graphite or carbon, wherein wet compression includescompressing or pressing a suspension of exfoliated graphite or carbondispersed in a liquid electrolyte intended for use in an aluminum cell.Any of the aforementioned electrolytes can be utilized in thissuspension. The electrolyte later becomes part of the electrolyte of theintended aluminum battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) Schematic drawing illustrating the processes for producingintercalated and/or oxidized graphite, subsequently exfoliated graphiteworms, and conventional paper, mat, film, and membrane of simplyaggregated graphite or graphene flakes/platelets;

FIG. 1(B) An SEM image of exfoliated carbon (exfoliated carbon worms);

FIG. 1(C) Another SEM image of graphite worms;

FIG. 1(D) Schematic drawing illustrating the approaches of producingthermally exfoliated graphite structures.

FIG. 1(E) A continuous process of producing recompressed exfoliatedgraphite, including feeding dry exfoliated graphite worms into the gapbetween a pair of two counter-rotating rollers or the gaps betweenseveral pairs of rollers.

FIG. 1(F) A schematic drawing to illustrate an example of a compressingand consolidating operation (using a mold cavity cell 302 equipped witha piston or ram 308) for forming a layer of highly compacted andoriented graphite flakes.

FIG. 1(G) Schematic drawing to illustrate another example of acompressing and consolidating operation (using a mold cavity cellequipped with a piston or ram) for forming a layer of highly compactedand oriented graphite flakes.

FIG. 2(A) Schematic of an aluminum secondary battery, wherein the anodelayer is a thin Al coating or Al foil and the cathode active materiallayer contains a layer of thermally exfoliated graphite/carbon; and

FIG. 2(B) schematic of an aluminum secondary battery cell having acathode active material layer composed of thermally exfoliatedgraphite/carbon material and an anode composed of Al metal coatingdeposited on surfaces of a nano-structured network of conductivefilaments.

FIG. 3(A) The discharge curves of three Al foil anode-based cells eachhaving a recompressed exfoliated graphite-based cathode: first onehaving an intercalated graphite that was thermally exfoliated andheavily recompressed to obtain an oriented structure having a specificsurface area (SSA)=23 m²/g, second one having an intercalated graphitethat was thermally exfoliated and moderately recompressed (specificsurface area=116 m²/g), and third one having an intercalated graphitethat was thermally exfoliated and moderately recompressed (specificsurface area=422 m²/g).

FIG. 3(B) The discharge curves of two Al foil anode-based cells eachhaving a cathode layer of lightly recompressed exfoliated graphitehaving a SSA of 802 and 1,013 m²/g, respectively.

FIG. 4 The specific capacity values of a wide variety of lightlyrecompressed, thermally exfoliated carbon or graphite materials plottedas a function of the specific surface area.

FIG. 5 The specific capacity values of three Al cells plotted as afunction of charge/discharge cycles: a cell containing a cathode layerof heavily recompressed exfoliated graphite (having an edge plane beingparallel to the separator and in ion-contact with the separator), a cellcontaining a cathode layer of lightly recompressed exfoliated graphite,and a cell containing a cathode of original graphite.

FIG. 6 The Ragone plots of four cells: a cell containing a cathode oforiginal artificial graphite, a cell containing a cathode of artificialgraphite that has been thermally exfoliated and lightly recompressed, acell containing a cathode of artificial graphite that has been thermallyexfoliated and heavily recompressed (flakes being properly aligned tohave a desired orientation, having an edge plane being parallel to theseparator) using a dry-pressing process, and a cell containing a cathodeof artificial graphite that has been thermally exfoliated and heavilyrecompressed (flakes being properly aligned to have a desiredorientation) using a wet-pressing process.

FIG. 7 The specific capacity values, plotted as a function ofcharge/discharge cycles, of two Al cells, one containing a cathode ofexfoliated graphite heavily recompressed to obtain desired graphiteorientation and the other containing a layer of thermally exfoliatedgraphite lightly recompressed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As schematically illustrated in the upper portion of FIG. 1(A), bulknatural graphite is a 3-D graphitic material with each graphite particlebeing composed of multiple grains (a grain being a graphite singlecrystal or crystallite) with grain boundaries (amorphous or defectzones) demarcating neighboring graphite single crystals. Each grain iscomposed of multiple graphene planes that are oriented parallel to oneanother. A graphene plane or hexagonal carbon atom plane in a graphitecrystallite is composed of carbon atoms occupying a two-dimensional,hexagonal lattice. In a given grain or single crystal, the grapheneplanes are stacked and bonded via van der Waal forces in thecrystallographic c-direction (perpendicular to the graphene plane orbasal plane). The inter-graphene plane spacing in a natural graphitematerial is approximately 0.3354 nm.

Artificial graphite materials also contain constituent graphene planes,but they have an inter-graphene planar spacing, d₀₀₂, typically from0.32 nm to 0.36 nm (more typically from 0.3339 to 0.3465 nm), asmeasured by X-ray diffraction. Many carbon or quasi-graphite materialsalso contain graphite crystals (also referred to as graphitecrystallites, domains, or crystal grains) that are each composed ofstacked graphene planes. These include meso-carbon mocro-beads (MCMBs),meso-phase carbon, soft carbon, hard carbon, coke (e.g. needle coke),carbon or graphite fibers (including vapor-grown carbon nano-fibers orgraphite nano-fibers), and multi-walled carbon nanotubes (MW-CNT). Thespacing between two graphene rings or walls in a MW-CNT is approximately0.27 to 0.42 nm. The most common spacing values in MW-CNTs are in therange of 0.32-0.35 nm, which do not strongly depend on the synthesismethod.

It may be noted that the “soft carbon” refers to a carbon materialcontaining graphite domains wherein the orientation of the hexagonalcarbon planes (or graphene planes) in one domain and the orientation inneighboring graphite domains are not too mis-matched from each other sothat these domains can be readily merged together when heated to atemperature above 2,000° C. (more typically above 2,500° C.). Such aheat treatment is commonly referred to as graphitization. Thus, the softcarbon can be defined as a carbonaceous material that can begraphitized. In contrast, a “hard carbon” can be defined as acarbonaceous material that contain highly mis-oriented graphite domainsthat cannot be thermally merged together to obtain larger domains; i.e.the hard carbon cannot be graphitized. Both hard carbon and soft carboncontain graphite domains that can be intercalated and thermallyexfoliated. The exfoliated carbon then can be recompressed to produce acathode layer having constituent graphite flakes being aligned.

The spacing between constituent graphene planes of a graphitecrystallite in a natural graphite, artificial graphite, and othergraphitic carbon materials in the above list can be expanded (i.e. thed₀₀₂ spacing being increased from the original range of 0.27-0.42 nm tothe range of 0.42-2.0 nm) using several expansion treatment approaches,including oxidation, fluorination, chlorination, bromination,iodization, nitrogenation, intercalation, combinedoxidation-intercalation, combined fluorination-intercalation, combinedchlorination-intercalation, combined bromination-intercalation, combinediodization-intercalation, or combined nitrogenation-intercalation of thegraphite or carbon material.

More specifically, due to the van der Waals forces holding the parallelgraphene planes together being relatively weak, natural graphite can betreated so that the spacing between the graphene planes can be increasedto provide a marked expansion in the c-axis direction. This results in agraphite material having an expanded spacing, but the laminar characterof the hexagonal carbon layers is substantially retained. Theinter-planar spacing (also referred to as inter-graphene spacing) ofgraphite crystallites can be increased (expanded) via severalapproaches, including oxidation, fluorination, and/or intercalation ofgraphite. This is schematically illustrated in FIG. 1(D). The presenceof an intercalant, oxygen-containing group, or fluorine-containing groupserves to increase the spacing between two graphene planes in a graphitecrystallite and weaken the van der Waals forces between graphene planes,enabling easier thermal exfoliation.

The inter-planar spaces between certain graphene planes may besignificantly increased (actually, exfoliated) if the graphite/carbonmaterial having expanded d spacing is exposed to a thermal shock (e.g.by rapidly placing this carbon material in a furnace pre-set at atemperature of typically 800-2,500° C.) without constraint (i.e. beingallowed to freely increase volume). Under these conditions, thethermally exfoliated graphite/carbon material appears like worms,wherein each graphite worm is composed of many graphite flakes remaininginterconnected (please see FIG. 1(C)). However, these graphite flakeshave inter-flake pores typically in the pore size range of 20 nm to 10μm.

In one process, graphite materials having an expanded inter-planarspacing are obtained by intercalating natural graphite particles with astrong acid and/or an oxidizing agent to obtain a graphite intercalationcompound (GIC) or graphite oxide (GO), as illustrated in FIG. 1(A). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes serves to increase the inter-graphenespacing, d₀₀₂, as determined by X-ray diffraction, thereby significantlyreducing the van der Waals forces that otherwise hold graphene planestogether along the c-axis direction. The GIC or GO is most oftenproduced by immersing natural graphite powder (100 in FIG. 1(A)) in amixture of sulfuric acid, nitric acid (an oxidizing agent), and anotheroxidizing agent (e.g. potassium permanganate or sodium perchlorate). Theresulting GIC (102) is actually some type of graphite oxide (GO)particles if an oxidizing agent is present during the intercalationprocedure. This GIC or GO is then repeatedly washed and rinsed in waterto remove excess acids, resulting in a graphite oxide suspension ordispersion, which contains discrete and visually discernible graphiteoxide particles dispersed in water.

Water may be removed from the suspension to obtain “expandablegraphite,” which is essentially a mass of dried GIC or dried graphiteoxide particles. The inter-graphene spacing, d₀₀₂, in the dried GIC orgraphite oxide particles is typically in the range of 0.42-2.0 nm, moretypically in the range of 0.5-1.2 nm. It may be noted than the“expandable graphite” is not “expanded graphite” (to be furtherexplained later). Graphite oxide can have an oxygen content of 2%-50% byweight, more typically 20%-40% by weight.

Upon exposure of expandable graphite to a temperature in the range oftypically 800-2,500° C. (more typically 900-1,050° C.) for approximately30 seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “exfoliated graphite” or “graphite worms”(104). Graphite worms are each a collection of exfoliated, but largelyun-separated graphite flakes that remain interconnected (FIG. 1(B) andFIG. 1(C)). In exfoliated graphite, individual graphite flakes (eachcontaining 1 to several hundred of graphene planes stacked together) arehighly spaced from one another, having a spacing of typically 2.0 nm-10μm. However, they remain physically interconnected, forming an accordionor worm-like structure.

Exfoliated graphite worms can be mechanically compressed to obtain“recompressed exfoliated graphite” for the purpose of densifying themass of exfoliated graphite worms, reducing inter-flake pore sizes orspaces, and aligning the orientation of the constituent flakes. (In someengineering applications, the graphite worms are extremely heavilycompressed to form flexible graphite sheets or foils 106 that typicallyhave a thickness in the range of 0.1 mm-0.5 mm.) In the instantinvention, as illustrated in the lower right portion of FIG. 1(D),exfoliated graphite worms are compressed to the extent that theconstituent graphite flakes are more or less parallel to one another andthe edges of these flakes define an edge plane of the resulting block orlayer of re-compressed graphite worms. Primary surfaces of some of thegraphite flakes (top or bottom surfaces) can constitute a flake surfaceplane (as opposed to the edge plane).

Alternatively, in graphite industry, one may choose to use alow-intensity air mill or shearing machine to simply break up thegraphite worms for the purpose of producing the so-called “expandedgraphite” flakes (108) which contain mostly graphite flakes or plateletsthicker than 100 nm (hence, not a nano material by definition). It isclear that the “expanded graphite” is not “expandable graphite” and isnot “exfoliated graphite worm” either. Rather, the “expandable graphite”can be thermally exfoliated to obtain “graphite worms,” which, in turn,can be subjected to mechanical shearing to break up the otherwiseinterconnected graphite flakes to obtain “expanded graphite” flakes.Expanded graphite flakes typically have the same or similar inter-planarspacing (typically 0.335-0.36 nm) of their original graphite. Expandedgraphite is not graphene either. Expanded graphite flakes have athickness typically greater than 100 nm; in contrast, graphene sheetstypically have a thickness smaller than 100 nm, more typically less than10 nm, and most typically less than 3 nm (single layer graphene is 0.34nm thick). In the present invention, expanded graphite flakes may alsobe compressed to form a layer of recompressed graphite having thedesired orientation.

Further alternatively, the exfoliated graphite or graphite worms may besubjected to high-intensity mechanical shearing (e.g. using anultrasonicator, high-shear mixer, high-intensity air jet mill, orhigh-energy ball mill) to form separated single-layer and multi-layergraphene sheets (collectively called NGPs, 112), as disclosed in ourU.S. application Ser. No. 10/858,814. Single-layer graphene can be asthin as 0.34 nm, while multi-layer graphene can have a thickness up to100 nm, but more typically less than 3 nm (commonly referred to asfew-layer graphene). Multiple graphene sheets or platelets may be madeinto a sheet of NGP paper (114) using a paper-making process.

It may be noted that the “expandable graphite” or graphite with expandedinter-planar spacing may also be obtained by forming graphite fluoride(GF), instead of GO. Interaction of F₂ with graphite in a fluorine gasat high temperature leads to covalent graphite fluorides, from (CF)_(n)to (C₂F)_(n), while at low temperatures graphite intercalation compounds(GIC) C_(x)F (2≤x≤24) form. In (CF)_(n) carbon atoms are sp3-hybridizedand thus the fluorocarbon layers are corrugated consisting oftrans-linked cyclohexane chairs. In (C₂F)_(n) only half of the C atomsare fluorinated and every pair of the adjacent carbon sheets are linkedtogether by covalent C—C bonds. Systematic studies on the fluorinationreaction showed that the resulting F/C ratio is largely dependent on thefluorination temperature, the partial pressure of the fluorine in thefluorinating gas, and physical characteristics of the graphiteprecursor, including the degree of graphitization, particle size, andspecific surface area. In addition to fluorine (F₂), other fluorinatingagents (e.g. mixtures of F₂ with Br₂, Cl₂, or I₂) may be used, althoughmost of the available literature involves fluorination with F₂ gas,sometimes in presence of fluorides.

We have observed that lightly fluorinated graphite, C_(x)F (2≤x≤24),obtained from electrochemical fluorination, typically has aninter-graphene spacing (d₀₀₂) less than 0.37 nm, more typically <0.35nm. Only when x in C_(x)F is less than 2 (i.e. 0.5≤x<2) can one observea d₀₀₂ spacing greater than 0.5 nm (in fluorinated graphite produced bya gaseous phase fluorination or chemical fluorination procedure). When xin C_(x)F is less than 1.33 (i.e. 0.5≤x<1.33) one can observe a d₀₀₂spacing greater than 0.6 nm. This heavily fluorinated graphite isobtained by fluorination at a high temperature (>>200° C.) for asufficiently long time, preferably under a pressure >1 atm, and morepreferably >3 atm. For reasons remaining unclear, electrochemicalfluorination of graphite leads to a product having a d spacing less than0.4 nm even though the product C_(x)F has an x value from 1 to 2. It ispossible that F atoms electrochemically introduced into graphite tend toreside in defects, such as grain boundaries, instead of between grapheneplanes and, consequently, do not act to expand the inter-graphene planarspacing.

The nitrogenation of graphite can be conducted by exposing a graphiteoxide material to ammonia at high temperatures (200-400° C.).Nitrogenation may also be conducted at lower temperatures by ahydrothermal method; e.g. by sealing GO and ammonia in an autoclave andthen increased the temperature to 150-250° C.

Compression or re-compression of exfoliated graphite worms or expandedgraphite flakes into a layer or block of recompressed exfoliatedgraphite having a preferred graphite flake orientation can beaccomplished by using several procedures, which can be classified intotwo broad categories: dry pressing/rolling or wet pressing/rolling. Thedry process entails mechanically pressing graphite worms or expandedgraphite flakes in one direction (uniaxial compression) without thepresence of a liquid medium. Alternatively, as schematically illustratedin FIG. 1(E), the process includes feeding dry exfoliated graphite worms30 into the gap between two counter-rotating rollers (e.g. 32 a and 32b) to form a slightly compressed layer of “re-compressed exfoliatedgraphite,” which are then further compressed to form a thinner layer offurther re-compressed exfoliated graphite (containing aligned graphiteflakes) by directing the material into the gap between another tworollers (e.g. 34 a and 34 b). If necessary, another pair or multiplepairs of rollers (e.g. 36 a and 36 b) can be implemented to furtherreduce the layer thickness and further improve the degree of flakeorientation, resulting in a layer 38 of relatively well-alignedrecompressed exfoliated graphite.

A layer of oriented, recompressed exfoliated graphite structure (ormultiple layers of such a structure stacked and/or bonded together) maybe cut and slit to produce a desired number of pieces of the oriented,recompressed exfoliated graphite structure. Assuming that each piece isa cube or tetragon, each cube will then have 4 graphite flake edgeplanes and 2 flake surface planes as illustrated in the bottom rightportion of FIG. 1(D). When such a piece is implemented as a cathodelayer, the layer can be positioned and aligned in such a manner that oneof the flake edge planes is substantially parallel to the anode layer orthe porous separator layer. This flake edge plane typically is veryclose to or actually in direct contact with the separator layer. Such anorientation is found to be conducive to entry and exiting of ionsinto/from the electrode, leading to significantly improved high-ratecapability and high power density.

It may be noted that the same procedures can be used to produce a wetlayer of recompressed exfoliated graphite provided the starting materialcontains graphite worms dispersed in a liquid medium. This liquid mediummay be simply water or solvent, which must be removed upon completion ofthe roll-pressing procedure. The liquid medium may be or may contain aresin binder that helps to bond together exfoliated graphite worms orflakes, although a resin binder is not required or desired.Alternatively and desirably, some amount of the liquid electrolyte(intended to become part of the electrolyte of the final aluminum cell)may be mixed with the exfoliated graphite worms or expanded graphiteflakes prior to being compressed or roll-pressed.

The present invention also provides a wet process for producing anelectrolyte-impregnated recompressed graphite structure for use as analuminum battery cathode layer. In a preferred embodiment, the wetprocess (method) comprises: (a) preparing a dispersion or slurry havingexfoliated graphite worms or expanded graphite flakes dispersed in aliquid or gel electrolyte; and (b) subjecting the suspension to a forcedassembly procedure, forcing the exfoliated graphite worms or expandedgraphite flakes to assemble into the electrolyte-impregnatedre-compressed graphite structure, wherein electrolyte resides in theinter-flake spaces in recompressed exfoliated graphite. The graphiteflakes of the exfoliated graphite or the expanded graphite flakes aresubstantially aligned along a desired direction. The recompressedgraphite structure has a physical density from 0.5 to 1.7 g/cm³ (moretypically 0.7-1.3 g/cm³) and a specific surface area from 20 to 1,500m²/g, when measured in a dried state of the recompressed graphitestructure with the electrolyte removed.

In some desired embodiments, the forced assembly procedure includesintroducing an exfoliated graphite suspension, having an initial volumeV₁, in a mold cavity cell and driving a piston into the mold cavity cellto reduce the suspension volume to a smaller value V₂, allowing excesselectrolyte to flow out of the cavity cell (e.g. through holes of themold cavity cell or of the piston) and aligning the multiple graphiteflakes along a direction at an angle from approximately 45° to 90°relative to the movement direction of the piston. It may be noted thatthe electrolyte used in this suspension becomes portion of theelectrolyte for the intended aluminum cell.

FIG. 1(F) provides a schematic drawing to illustrate an example of acompressing and consolidating operation (using a mold cavity cell 302equipped with a piston or ram 308) for forming a layer of highlycompacted and oriented graphite flakes 314. Contained in the chamber(mold cavity cell 302) is a suspension (or slurry) that is composed ofgraphite flakes 304 randomly dispersed in a liquid or gel electrolyte306. As the piston 308 is driven downward, the volume of the suspensionis decreased by forcing excess liquid electrolyte to flow through minutechannels 312 on a mold wall or through small channels 310 of the piston.These small channels can be present in any or all walls of the moldcavity and the channel sizes can be designed to permit permeation of theelectrolyte species, but not the solid graphite flakes. The excesselectrolyte is shown as 316 a and 316 b on the right diagram of FIG.1(E). As a result of this compressing and consolidating operation,graphite flakes 314 are aligned parallel to the bottom plane orperpendicular to the layer thickness direction.

Shown in FIG. 1(G) is a schematic drawing to illustrate another exampleof a compressing and consolidating operation (using a mold cavity cellequipped with a piston or ram) for forming a layer of highly compactedand oriented graphite flakes 320. The piston is driven downward alongthe Y-direction. The graphite flakes are aligned on the X-Z plane andperpendicular to X-Y plane (along the Z- or thickness direction). Thislayer of oriented graphite flakes can be attached to a current collector(e.g. graphene mat) that is basically represented by the X-Y plane. Inthe resulting electrode, graphite flakes are aligned perpendicular tothe current collector. Such an orientation is conducive to a faster ionintercalation into and out of the spaces between graphite flakes and,hence, leads to a higher power density as compared to the correspondingelectrode featuring graphite flakes being aligned parallel to thecurrent collector plane.

The configuration of an aluminum secondary battery is now discussed asfollows:

An aluminum secondary battery includes a positive electrode (cathode), anegative electrode (anode), and an electrolyte including an aluminumsalt and a solvent. The anode can be a thin foil or film of aluminummetal or aluminum metal alloy (e.g. left-hand side of FIG. 2(A)). Theanode can be composed of particles, fibers, wires, tubes, or discs of Almetal or Al metal alloy that are packed and bonded together by a binder(preferably a conductive binder) to form an anode layer.

A desirable anode layer structure is composed of a network ofelectron-conducting pathways (e.g. mat of graphene sheets, carbonnano-fibers, or carbon-nanotubes) and a thin layer of aluminum metal oralloy coating deposited on surfaces of this conductive network structure(e.g. left-hand side of FIG. 2(B)). Such an integrated nano-structuremay be composed of electrically conductive nanometer-scaled filamentsthat are interconnected to form a porous network of electron-conductingpaths comprising interconnected pores, wherein the filaments have atransverse dimension less than 500 nm. Such filaments may comprise anelectrically conductive material selected from the group consisting ofelectro-spun nano fibers, vapor-grown carbon or graphite nano fibers,carbon or graphite whiskers, carbon nano-tubes, nano-scaled grapheneplatelets, metal nano wires, and combinations thereof. Such anano-structured, porous supporting material for aluminum cansignificantly improve the aluminum deposition-dissolution kinetics,enabling high-rate capability of the resulting aluminum secondary cell.

Illustrated in FIG. 2(A) is a schematic of an aluminum secondarybattery, wherein the anode layer is a thin Al coating or Al foil and thecathode active material layer contains a layer of thermally exfoliatedgraphite/carbon material that has been recompressed. Alternatively, FIG.2(B) shows a schematic of an aluminum secondary battery cell wherein theanode layer is composed of a thin coating of aluminum metal or aluminumalloy supported on surfaces of a network of conductive filaments and thecathode active material layer contains a layer of thermally exfoliatedgraphite/carbon material that has been recompressed. The layer or blockof oriented, recompressed exfoliated graphite/carbon has a flake edgeplane facing the separator and substantially parallel to the separatorlayer.

The composition of the electrolyte, which functions as anion-transporting medium for charge-discharge reaction, has a greateffect on battery performance. To put aluminum secondary batteries topractical use, it is necessary to allow aluminum deposition-dissolutionreaction to proceed smoothly and sufficiently even at relatively lowtemperature (e.g., room temperature). In conventional aluminum secondarybatteries, however, aluminum deposition-dissolution reaction can proceedsmoothly and sufficiently only at relatively high temperature (e.g., 50°C. or higher), and the efficiency of the reaction is also low. Theelectrolyte for use in an aluminum secondary battery can include analuminum salt, alkyl sulfone, and a solvent with a dielectric constantof 20 or less so that the electrolyte can operate at a lower temperature(e.g. room temperature) at which aluminum deposition-dissolutionreaction proceeds.

Aqueous electrolytes that can be used in the aluminum secondarybatteries include aluminum salts dissolved in water; for instance,AlCl₃-6H₂O, CrCl₃-6H₂O, and Al(NO₃)₃ in water. Alkaline solutions, suchas KOH and NaOH in water, may also be used.

Organic electrolytes for use in aluminum secondary batteries includevarious electrolytes with g-butyrolactone (BLA) or acetonitrile (ACN) assolvent; e.g. AlCl₃/KCl salts in BLA or (C₂H₅)₄NClxH₂O (TEAC) in ACN.Also included are concentrated aluminum triflate-based electrolyte, abath of aluminum chloride and lithium aluminum hydride dissolved indiethyl ether, and LiAlH₄ and AlCl₃ in tetrahydrofuran. For example,alkyl sulfone such as dimethylsulfone may be used, along with an organicsolvent such as a cyclic or chain carbonate or a cyclic or chain ethercan be used. In order to reduce polarization during discharge, analuminum salt such as aluminum chloride and an organic halide such astrimethylphenylammonium chloride may be used together in theelectrolyte. For this salt mixture, an organic solvent such as1,2-dichloroethane may be used.

Another type of electrolyte capable of reversible aluminumelectrochemistry is molten salt eutectics. These are typically composedof aluminum chloride, sodium chloride, potassium chloride and lithiumchloride in some molar ratio. Useful molten salt electrolytes includeAlCl₃—NaCl, AlCl₃-(LiCl—KCl), and AlCl₃—KCl—NaCl mixtures. Among thesealkali chloroaluminate melts, binary NaCl—AlCl₃ and ternaryNaCl—KCl—ACl₃ systems are the most widely used molten salts fordeveloping aluminum batteries. In these systems the melts with molarratio of MCl/AlCl₃ (where M is commonly Na and/or K) larger than unityare defined as basic, whereas those with molar ratio less than unity asacidic. In an acidic melt, Al₂Cl₇ ⁻ is the major anion species. As theacidity (AlCl₃ content) of the melt decreases, AlCl₄ ⁻ becomes the majorspecies.

A special class of molten salt for use in an aluminum secondary batteryis room temperature molten salts (ionic liquids). For instance, a usefulionic liquid electrolyte solution is aluminum chloride mixed in1-ethyl-3-methylimidazolium chloride (AlCl₃:EMIC). Commerciallyavailable 1-ethyl-3-methylimidazolium chloride may be purified byrecrystallization from ethyl acetate and acetonitrile. Aluminum chloridemay be further purified by triple sublimation. The ionic liquid may beprepared by slowly mixing molar equivalent amounts of both salts.Further, AlCl₃ was then added to the equimolar mix until a concentrationof 1M AlCl₃ was obtained. Desirably, this concentration corresponds to amolar ratio of 1.2:1, AlCl₃:EMIC.

Aluminum chloride (AlCl₃) also forms room temperature electrolytes withorganic chlorides, such as n-butyl-pyridinium-chloride (BuPyCl),1-methyl-3-ethylimidazolium-chloride (MEICl), and2-dimethyl-3-propylimidazolium-chloride. The molten mixture of1,4-dimethyl-1,2,4-triazolium chloride (DMTC) and AlCl₃ may also be usedas the secondary battery electrolyte.

This invention is directed at the cathode active layer (positiveelectrode layer) containing a high-capacity cathode material for thealuminum secondary battery. The invention also provides such a batterybased on an aqueous electrolyte, a non-aqueous electrolyte, a moltensalt electrolyte, a polymer gel electrolyte (e.g. containing an aluminumsalt, a liquid, and a polymer dissolved in the liquid), or an ionicliquid electrolyte. The shape of an aluminum secondary battery can becylindrical, square, button-like, etc. The present invention is notlimited to any battery shape or configuration.

The following examples are used to illustrate some specific detailsabout the best modes of practicing the instant invention and should notbe construed as limiting the scope of the invention.

Example 1: Oxidation of Graphite and Thermal Exfoliation of OxidizedGraphite

Natural flake graphite, nominally sized at 45 μm, provided by AsburyCarbons (405 Old Main St., Asbury, N.J. 08802, USA) was milled to reducethe size to approximately 14 μm (Sample 1a). The chemicals used in thepresent study, including fuming nitric acid (>90%), sulfuric acid(95-98%), potassium chlorate (98%), and hydrochloric acid (37%), werepurchased from Sigma-Aldrich and used as received. Graphite oxide (GO)samples were prepared according to the following procedure:

Sample 1A: A reaction flask containing a magnetic stir bar was chargedwith sulfuric acid (176 mL) and nitric acid (90 mL) and cooled byimmersion in an ice bath. The acid mixture was stirred and allowed tocool for 15 min, and graphite (10 g) was added under vigorous stirringto avoid agglomeration. After the graphite powder was well dispersed,potassium chlorate (110 g) was added slowly over 15 min to avoid suddenincreases in temperature. The reaction flask was loosely capped to allowevolution of gas from the reaction mixture, which was stirred for 24hours at room temperature. On completion of the reaction, the mixturewas poured into 8 L of deionized water and filtered. The GO wasre-dispersed and washed in a 5% solution of HCl to remove sulphate ions.The filtrate was tested intermittently with barium chloride to determineif sulphate ions are present. The HCl washing step was repeated untilthis test was negative. The GO was then washed repeatedly with deionizedwater until the pH of the filtrate was neutral. The GO slurry wasspray-dried and stored in a vacuum oven at 60° C. until use.

Sample 1B: The same procedure as in Sample 1A was followed, but thereaction time was 48 hours.

Sample 1C: The same procedure as in Sample 1A was followed, but thereaction time was 96 hours.

X-ray diffraction studies showed that after a treatment of 24 hours, asignificant proportion of graphite has been transformed into graphiteoxide. The peak at 20=26.3 degrees, corresponding to an inter-planarspacing of 0.335 nm (3.35 Å) for pristine natural graphite wassignificantly reduced in intensity after a deep oxidation treatment for24 hours and a peak typically near 20=9-14 degrees (depending upondegree of oxidation) appeared. In the present study, the curves fortreatment times of 48 and 96 hours are essentially identical, showingthat essentially all of the graphite crystals have been converted intographite oxide with an inter-planar spacing of 6.5-7.5 Å (the 26.3degree peak has totally disappeared and a peak of approximately at2θ=11.75-13.7 degrees appeared).

Samples 1A, 1B, and 1C were then subjected to unconstrained thermalexfoliation (1,050° C. for 2 minutes) to obtain thermally exfoliatedgraphite worms. The graphite worms were compressed into layers oforiented, recompressed exfoliated graphite having physical densityranging from approximately 0.5 to 1.75 g/cm³, using both dry compressionand wet compression procedures.

Example 2: Oxidation, Intercalation and Thermal Exfoliation of VariousGraphitic Carbon and Graphite Materials

Samples 2A, 2B, 2C, and 2D were prepared according to the same procedureused for Sample 1B, but the starting graphite materials were pieces ofhighly oriented pyrolytic graphite (HOPG), graphite fiber, graphiticcarbon nano-fiber, and spheroidal graphite, respectively. After theexpansion treatment, their final inter-planar spacings are 6.6 Å, 7.3 Å,7.3 Å, and 6.6 Å, respectively. They were subsequently thermallyexfoliated and recompressed to obtain samples of various controlleddensities, specific surface areas, and degrees of orientation.

Example 3: Preparation of Graphite Oxide (GO) Using a Modified Hummers'Method and Subsequent Thermal Exfoliation

Graphite oxide (Sample 3A) was prepared by oxidation of natural graphiteflakes with sulfuric acid, sodium nitrate, and potassium permanganateaccording to the method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9,1957]. In this example, for every 1 gram of graphite, we used a mixtureof 22 ml of concentrated sulfuric acid, 2.8 grams of potassiumpermanganate, and 0.5 grams of sodium nitrate. The graphite flakes wereimmersed in the mixture solution and the reaction time was approximatelyone hour at 35.degree. C. It is important to caution that potassiumpermanganate should be gradually added to sulfuric acid in awell-controlled manner to avoid overheat and other safety issues. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The sample was then washed repeatedly with deionized wateruntil the pH of the filtrate was approximately 5. The slurry wasspray-dried and stored in a vacuum oven at 60° C. for 24 hours. Theinterlayer spacing of the resulting laminar graphite oxide wasdetermined by the Debye-Scherrer X-ray technique to be approximately0.73 nm (7.3 Å). Some of the powder was subsequently exfoliated in afurnace, pre-set at 950-1,100° C., for 2 minutes to obtain thermallyexfoliated graphite worms. The graphite worms were re-compressed usingboth the wet and dry press-rolling procedures to obtain oriented,recompressed exfoliated graphite worms.

Example 4: Oxidation and Thermal Exfoliation of Meso-Carbon Micro-Beads(MCMBs)

Oxidized carbon beads (Sample 4A) were prepared by oxidation ofmeso-carbon micro-beads (MCMBs) according to the same procedure used inExample 3. MCMB microbeads (Sample 4a) were supplied by China SteelChemical Co. This material has a density of about 2.24 g/cm³; an averageparticle of 16 microns and an inter-planar distance of about 0.336 nm.After deep oxidation/intercalation treatment, the inter-planar spacingin the resulting graphite oxide micro-beads is approximately 0.76 nm.The treated MCMBs were then thermally exfoliated at 900° C. for 2minutes to obtain exfoliated carbon, which also showed a worm-likeappearance (herein referred to as “exfoliated carbon”, “carbon worms,”or “exfoliated carbon worms”). The carbon worms were then roll-pressedto different extents to obtain recompressed exfoliated carbon havingdifferent densities, specific surface areas, and degrees of orientation.

Example 5: Bromination and Fluorination of Carbon Fibers and ThermalExfoliation

Graphitized carbon fiber (Sample 5a), having an inter-planar spacing of3.37 Å (0.337 nm) and a fiber diameter of 10 μm was first halogenatedwith a combination of bromine and iodine at temperatures ranging from75° C. to 115° C. to form a bromine-iodine intercalation compound ofgraphite as an intermediate product. The intermediate product was thenreacted with fluorine gas at temperatures ranging from 275° C. to 450°C. to form the CF_(y). The value of y in the CF_(y) samples wasapproximately 0.6-0.9. X-ray diffraction curves typically show theco-existence of two peaks corresponding to 0.59 nm and 0.88 nm,respectively. Sample 5A exhibits substantially 0.59 nm peak only andSample 5B exhibits substantially 0.88 nm peak only. Some of powders werethermally exfoliated and then re-compressed to obtain oriented,recompressed exfoliated graphite.

Example 6: Fluorination and Thermal Exfoliation of Carbon Fibers

A CF_(0.68) sample obtained in EXAMPLE 5 was exposed at 250° C. and 1atmosphere to vapors of 1,4-dibromo-2-butene (BrH₂C—CH═.CH—CH₂Br) for 3hours. It was found that two-thirds of the fluorine was lost from thegraphite fluoride sample. It is speculated that 1,4-dibromo-2-buteneactively reacts with graphite fluoride, removing fluorine from thegraphite fluoride and forming bonds to carbon atoms in the graphitelattice. The resulting product (Sample 6A) is mixed halogenatedgraphite, likely a combination of graphite fluoride and graphitebromide. Some of powders were thermally exfoliated to obtain exfoliatedcarbon fibers, which were then roll-pressed to obtain oriented,recompressed exfoliated graphite worms.

Example 7: Fluorination and Thermal Exfoliation of Graphite

Natural graphite flakes, a sieve size of 200 to 250 mesh, were heated invacuum (under less than 10⁻² mmHg) for about 2 hours to remove theresidual moisture contained in the graphite. Fluorine gas was introducedinto a reactor and the reaction was allowed to proceed at 375° C. for120 hours while maintaining the fluorine pressure at 200 mmHg. This wasbased on the procedure suggested by Watanabe, et al. disclosed in U.S.Pat. No. 4,139,474. The powder product obtained was black in color. Thefluorine content of the product was measured as follows: The product wasburnt according to the oxygen flask combustion method and the fluorinewas absorbed into water as hydrogen fluoride. The amount of fluorine wasdetermined by employing a fluorine ion electrode. From the result, weobtained a GF (Sample 7A) having an empirical formula (CF_(0.75))_(n).X-ray diffraction indicated a major (002) peak at 20=13.5 degrees,corresponding to an inter-planar spacing of 6.25 Å. Some of the graphitefluoride powder was thermally exfoliated to form graphite worms, whichwere then roll-pressed.

Sample 7B was obtained in a manner similar to that for Sample 7A, but ata reaction temperature of 640° C. for 5 hours. The chemical compositionwas determined to be (CF_(0.93))_(n). X-ray diffraction indicated amajor (002) peak at 20=9.5 degrees, corresponding to an inter-planarspacing of 9.2 Å. Some of the graphite fluoride powder was thermallyexfoliated to form graphite worms, which were then roll-pressed toproduce recompressed exfoliated graphite material.

Example 8: Preparation and Testing of Various Aluminum Cells

The exfoliated carbon/graphite materials prepared in Examples 1-7 wereseparately made into a cathode layer and incorporated into an aluminumsecondary battery. Two types of Al anode were prepared. One was Al foilhaving a thickness from 16 μm to 300 μm. The other was Al thin coatingdeposited on surfaces of conductive nano-filaments (e.g. CNTs) orgraphene sheets that form an integrated 3D network ofelectron-conducting pathways having pores and pore walls to accept Al orAl alloy. Either the Al foil itself or the integrated 3D nano-structurealso serves as the anode current collector.

Cyclic voltammetry (CV) measurements were carried out using an Arbinelectrochemical workstation at a typical scanning rate of 0.5-50 mV/s.In addition, the electrochemical performances of various cells were alsoevaluated by galvanostatic charge/discharge cycling at a current densityfrom 50 mA/g to 10 A/g. For long-term cycling tests, multi-channelbattery testers manufactured by LAND were used.

FIG. 3(A) shows the discharge curves of three Al foil anode-based cellseach having a recompressed exfoliated graphite-based cathode: first onehaving an intercalated graphite that was thermally exfoliated andheavily recompressed to obtain an oriented structure having a specificsurface area (SSA)=23 m²/g, second one having an intercalated graphitethat was thermally exfoliated and moderately recompressed (specificsurface area=116 m²/g), and third one having an intercalated graphitethat was thermally exfoliated and moderately recompressed (specificsurface area=422 m²/g). The electrolyte used was aluminum chloride mixedin 1-ethyl-3-methylimidazolium chloride (AlCl₃:EMIC molar ratio=3.5/1).These data indicate that the three battery cells all exhibit an initialplateau voltage, but the length of this plateau varied with differentthermal exfoliation and recompression conditions. The cathode layerobtained from heavy recompression exhibits the longest plateau; thismechanism quite likely corresponds to intercalation of Al³⁺, AlCl₄ ⁻,and/or Al₂Cl₇ ⁻ ions into the nano pores or interstitial spaces betweengraphite flakes or between the constituent graphene planes of theseflakes, further explained below:

In the discharge process, Al metal is oxidized and released from Al foilto form Al³⁺ ions. Under the influence of the electric field, Al³⁺ ionsmove to the cathode side. Then, Al³⁺ ions and aluminum chloridecoordination anions [Al_(a)Cl_(b)]⁻ can simultaneously intercalate intothe graphite layers, forming Al_(x)Cl_(y). The intercalated Al_(x)Cl_(y)and neighboring graphite layers interact with each other by van derWaals' forces. During the charge process, the electrochemical reactionsare reversed.

In the ionic liquid-based electrolyte, the existing coordination ionsare AlCl₄ ⁻ or Al₂Cl₇ ⁻, and thus the intercalated coordination ion[Al_(a)Cl_(b)]⁻ might be AlCl₄ ⁻ or Al₂Cl₇ ⁻ or a mixture thereof. Basedon the above assumption, the electrode reactions for both the anode andcathode may be described as follows:

In the charge process,

Al³⁺+3e ⁻→Al (anode)  (1)

Al_(x)Cl_(y) −e ⁻→Al³⁺+[Al_(a)Cl_(b)]⁻ (cathode)  (2)

At the anode, during battery charging, Al₂Cl₇ ⁻ ions can react withelectrons to form AlCl₄ ⁻ anions and Al. At the cathode, desorption ofEMI⁺ ions from graphite surfaces may also occur.

In the discharge process,

Al−3e ⁻→Al³⁺ (anode)  (3)

Al³⁺+[Al_(a)Cl_(b)]⁻ +e ⁻→Al_(x)Cl_(y) (cathode)  (4)

It appears that the strategy of heavily recompressing the exfoliatedgraphite can result in the inter-planar spaces between graphene planesto being smaller than 20 nm, preferably and typically smaller than 10 nm(having the inter-flake pores <10 nm) enables massive amounts of theions to “intercalate” into these confined spaces at a reasonably highvoltage (2.2 vs. Al/Al³⁺). Such an intercalation at a relatively highvoltage over a long plateau range (large specific capacity, up to250-350 mAh/g, depending on pore sizes) implies a high specific energyvalue (obtained by integrating the voltage curve over the specificcapacity range) based on the cathode active material weight.

When the intercalated/oxidized/fluorinated graphite/carbon materialswere subjected to free thermal exfoliation (without constraint), many ofthe initially expanded interstitial spaces got further expanded andexfoliated to produce exfoliated graphite/carbon worms having thingraphite flakes with inter-flake pores in the range of 20 nm to 10 μm.This would leave behind lesser amounts of the tight inter-planar spaces(approximately 0.336 nm) between hexagonal carbon atom planes toaccommodate intercalated ions of Al³⁺, AlCl₄ ⁻, and/or A₂Cl₇ ⁺. However,more surface areas become accessible to liquid electrolyte and thesurfaces would become available for ion adsorption/desorption and/orsurface redox reactions, leading to supercapacitor-type behaviors(electric double layer capacitance, EDLC, or redox pseudo-capacitance).The slopping voltage curves after a short initial plateau regime for twofreely exfoliated and moderately recompressed samples are shown in FIG.3(A), one having a specific surface area (SSA) of 116 and 422 m²/g,respectively. A larger SSA leads to a shorter plateau length and alonger slopping voltage curve.

We have observed that the plateau regime totally disappears when thefreely exfoliated graphite/carbon material is lightly recompressed toexhibit a SSA that exceeds approximately 800-900 m²/g. For instance,FIG. 3(B) shows the discharge curves of two samples having a SSA of 802and 1,013 m²/g, respectively. None of them have a plateau regime. Theslopping voltage curves during battery discharge are likely due todesorption of AlCl₄ ⁻ anions and EMI⁺ ions from graphite surfaces. Thedegree of compression and, thus, the specific surface area (a reflectionof inter-flake spaces) have a significant impact on the specificcapacity of recompressed exfoliated graphite/carbon materials. FIG. 4indicates that heavy recompression (as reflected by a low specificsurface area) and graphite flake edge plane orientation lead to veryhigh specific capacity of the cathode layer of recompressed exfoliatedgraphite having an edge plane aligned parallel to the separator andfacing the separator. It seems that recompression tends to reduce theinter-flake spaces down to 2-20 nm range, enabling anintercalation/de-intercalation type charge storage mechanism, and thatthe flake edges, if properly oriented, enable easier/faster and fullentry of ions into inter-flake spaces.

In summary, the charge or discharge of the invented cathode layer caninvolve several charge storage mechanisms. Not wishing to be bound bytheory, but we believe that the main mechanisms at the cathode duringbattery charging are (1) desorption of EMI⁺ ions from graphite surfaces,(2) de-intercalation by Al³⁺ and AlCl₄ ⁻ from the inter-planar spaces,and (3) desorption of AlCl₄ ⁻ and Al₂Cl₇ ⁻ ions from graphite flakesurfaces. At the anode, during battery charging, Al₂Cl₇ ⁻ ions can reactwith electrons to form AlCl₄ ⁻ anions and Al, wherein AlCl₄ ⁻ anionsmove toward the cathode and Al deposits on Al foil or surface of theanode current collector. The Al³⁺ ions released from the cathode mayalso react with electrons to form Al metal atoms that re-deposit onto Alfoil surface or the surface of an anode current collector. Some EMI⁺ions may form electric double layers near the anode surfaces. The aboveprocesses are reversed when the battery is discharged. Differentmechanisms can dominate in different regimes of the charge-dischargecurves for the cathodes having different amounts of controlledinterstitial spaces (2-20 nm) and inter-flake pores (20 nm-10 μm)prepared by different procedures (different extents of recompressionafter exfoliation).

FIG. 5 shows the specific capacity values of three Al cells plotted as afunction of charge/discharge cycles: a cell containing a cathode layerof heavily recompressed exfoliated graphite (properly oriented), a cellcontaining a cathode layer of lightly recompressed exfoliated graphite,and a cell containing a cathode of original graphite. These datademonstrate that, compared with the original graphite, anintercalated/fluorinated/oxidized carbon/graphite material, ifexfoliated and heavily recompressed to produce an oriented structurehaving a graphite edge plane parallel to the separator, imparts asignificantly higher charge storage capacity to an aluminum-ion battery.The procedure of thermally exfoliating theintercalated/fluorinated/oxidized carbon/graphite material, followed bylight recompression, can also store more charges as compared to theoriginal graphite-based cathode layer; but the mechanism is dominatedmainly by surface adsorption/desorption or surface redox. Both types ofAl cells (heavy or light recompression) exhibit very stable cyclingbehaviors. This is further evidenced in FIG. 7 that shows thecharge-discharge capacity values remaining substantially unchanged up to8,000 cycles. The presently invented aluminum cells exhibit somesupercapacitor-like behavior (having long cycle life) and some lithiumion battery-like behavior (moderate energy density).

FIG. 6 shows the Ragone plots of four cells: a cell containing a cathodeof original artificial graphite, a cell containing a cathode ofartificial graphite that has been thermally exfoliated and lightlyrecompressed, a cell containing a cathode of artificial graphite thathas been thermally exfoliated and heavily recompressed (properlyoriented) using a dry-pressing process, and a cell containing a cathodeof artificial graphite that has been thermally exfoliated and heavilyrecompressed (properly oriented) using a wet-pressing process. There areseveral unexpected results. The first is the observation that the Alcell featuring a cathode of artificial graphite that has been thermallyexfoliated and heavily recompressed (properly oriented) using awet-pressing process exhibits the highest cell-level specific energy,reaching as high as 182 Wh/kg, comparable to that of the lithium-ionbattery and 20 times higher than that of a supercapacitor. The powerdensity, 3,435 W/kg, is as high as that of the current supercapacitorand significantly higher than the power density (typically <500 W/kg) ofthe lithium-ion battery. The wet process having an intended liquidelectrolyte as the liquid medium in the graphite worm suspension enablesthe liquid electrolyte to reach all places where electrolyte is needed.

The second is the notion that the Al cell featuring a cathode ofartificial graphite that has been thermally exfoliated and heavilyrecompressed (properly oriented) using a dry-pressing process deliversboth a high power density (2,628 W/kg) and high energy density of (162Wh/kg) as well. The lightly recompressed graphite worms exhibit lowerpower density even though the cathode layer has a much higher specificsurface area (typically 200-1,500 m²/g) as compared to the highlyrecompressed one (typically 20-200 m²/g). All cathode layers containingexfoliated graphite worms, recompressed lightly or heavily, exhibitbetter energy density and power density than does the layer containingoriginal graphite.

We claim:
 1. An aluminum secondary battery comprising an anode, acathode, a porous separator electronically separating said anode andsaid cathode, and an electrolyte in ionic contact with said anode andsaid cathode to support reversible deposition and dissolution ofaluminum at said anode, wherein said anode contains aluminum metal or analuminum metal alloy as an anode active material and said cathodecomprises a layer of recompressed exfoliated graphite or carbon materialthat is oriented in such a manner that said layer has a graphite edgeplane in direct contact with said electrolyte and facing said separator.2. The aluminum secondary battery of claim 1, wherein said exfoliatedcarbon or graphite material in said cathode active layer is selectedfrom a thermally exfoliated product of meso-phase pitch, meso-phasecarbon, meso carbon micro-beads (MCMB), coke particles, expandedgraphite flakes, artificial graphite particles, natural graphiteparticles, highly oriented pyrolytic graphite, soft carbon particles,hard carbon particles, multi-walled carbon nanotubes, carbonnano-fibers, carbon fibers, graphite nano-fibers, graphite fibers,carbonized polymer fibers, or a combination thereof.
 3. The aluminumsecondary battery of claim 1, wherein said layer of recompressedexfoliated graphite or carbon material has a physical density from 0.5to 1.8 g/cm³ and has meso-scaled pores having a pore size from 2 nm to50 nm.
 4. The aluminum secondary battery of claim 1, wherein said layerof recompressed exfoliated graphite or carbon material has a physicaldensity from 1.1 to 1.8 g/cm³ and has pores having a pore size from 2 nmto 5 nm.
 5. The aluminum secondary battery of claim 1, wherein saidlayer of recompressed exfoliated graphite or carbon material has aspecific surface area from 20 m²/g to 1,500 m²/g.
 6. The aluminumsecondary battery of claim 1, further comprising an anode currentcollector supporting said aluminum metal or aluminum metal alloy orfurther comprising a cathode current collector supporting said layer ofrecompressed exfoliated graphite or carbon material.
 7. The aluminumsecondary battery of claim 6, wherein said anode current collectorcontains an integrated nano-structure of electrically conductivenanometer-scaled filaments that are interconnected to form a porousnetwork of electron-conducting paths comprising interconnected pores,wherein said filaments have a transverse dimension less than 500 nm. 8.The aluminum secondary battery of claim 7, wherein said filamentscomprise an electrically conductive material selected from the groupconsisting of electro-spun nano fibers, vapor-grown carbon or graphitenano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaledgraphene platelets, metal nano wires, and combinations thereof.
 9. Thealuminum secondary battery of claim 1, wherein said electrolyte isselected from an aqueous electrolyte, organic electrolyte, molten saltelectrolyte, ionic liquid electrolyte, or a combination thereof.
 10. Thealuminum secondary battery of claim 1, wherein said electrolyte containsAlF₃, AlCl₃, AlBr₃, AlI₃, AlF_(x)Cl_((3-x)), AlBr_(x)Cl_((3-x)),AlI_(x)Cl_((3-x)), or a combination thereof, wherein x is from 0.01 to2.0.
 11. The aluminum secondary battery of claim 1, wherein saidelectrolyte contains an ionic liquid that contains an aluminum saltmixed with an organic chloride selected from n-butyl-pyridinium-chloride(BuPyCl), 1-methyl-3-ethylimidazolium-chloride (MEICl),2-dimethyl-3-propylimidazolium-chloride, 1,4-dimethyl-1,2,4-triazoliumchloride (DMTC), or a mixture thereof.
 12. The aluminum secondarybattery of claim 1, wherein the electrolyte also supports reversibleintercalation and de-intercalation of ions at the cathode, wherein saidions include cations, anions, or both.
 13. The aluminum secondarybattery of claim 1, wherein said layer of recompressed exfoliatedgraphite or carbon material operates as a cathode current collector tocollect electrons during a discharge of said aluminum secondary batteryand wherein said battery contains no separate or additional cathodecurrent collector.
 14. The aluminum secondary battery of claim 1,wherein said layer of recompressed exfoliated graphite or carbonmaterial further comprises an electrically conductive binder materialwhich bonds said exfoliated carbon or graphite material together to forma cathode electrode layer.
 15. The aluminum secondary battery of claim14, wherein said electrically conductive binder material comprises coaltar pitch, petroleum pitch, meso-phase pitch, a conducting polymer, apolymeric carbon, or a derivative thereof.
 16. The aluminum secondarybattery of claim 1, wherein said battery has an average dischargevoltage no less than 1.5 volt and a cathode specific capacity greaterthan 100 mAh/g based on a total cathode active layer weight.
 17. Thealuminum secondary battery of claim 1, wherein said battery has anaverage discharge voltage no less than 1.5 volt and a cathode specificcapacity greater than 200 mAh/g based on a total cathode active layerweight.
 18. The aluminum secondary battery of claim 1, wherein saidbattery has an average discharge voltage no less than 2.0 volts and acathode specific capacity greater than 100 mAh/g based on a totalcathode active layer weight.
 19. The aluminum secondary battery of claim1, wherein said battery has an average discharge voltage no less than2.0 volts and a cathode specific capacity greater than 200 mAh/g basedon a total cathode active layer weight.
 20. A method of manufacturing analuminum secondary battery, comprising: (a) providing an anodecontaining aluminum metal or an aluminum alloy; (b) providing a cathodecontaining a layer of recompressed exfoliated carbon or graphitematerial; and (c) providing a porous separator electronically separatingsaid anode and said cathode and an electrolyte capable of supportingreversible deposition and dissolution of aluminum at the anode andreversible adsorption/desorption and/or intercalation/de-intercalationof ions at the cathode; wherein said layer of recompressed exfoliatedcarbon or graphite material is oriented in such a manner that said layerhas a graphite edge plane in direct contact with said electrolyte andfacing or contacting said separator.
 21. The method of claim 20, furtherincluding providing a porous network of electrically conductivenano-filaments to support said aluminum metal or aluminum alloy at theanode.
 22. The method of claim 20, wherein said electrolyte contains anaqueous electrolyte, an organic electrolyte, a molten salt electrolyte,or an ionic liquid.
 23. The method of claim 20, wherein providing acathode contains subjecting a carbon or graphite material to anexpansion treatment selected from an oxidation, fluorination,bromination, chlorination, nitrogenation, intercalation, combinedoxidation-intercalation, combined fluorination-intercalation, combinedbromination-intercalation, combined chlorination-intercalation, orcombined nitrogenation-intercalation, followed by thermal exfoliation ata temperature from 100° C. to 2,500° C.
 24. The method of claim 20,wherein said procedure of providing the cathode includes compressingexfoliated graphite or carbon using a wet compression or dry compressionto align constituent graphite flakes of said exfoliated graphite orcarbon.
 25. The method of claim 20, wherein said procedure of providingthe cathode includes compressing exfoliated graphite or carbon using awet compression to align constituent graphite flakes of said exfoliatedgraphite or carbon, wherein said wet compression includes compressing orpressing a suspension of exfoliated graphite or carbon dispersed in aliquid electrolyte for an aluminum cell.